In 1956 in Taiwan, a new rice variety, “Taichung Native 1”, gave a high yield that had not been observed with conventional indica varieties. “Taichung Native 1” was bred from a cross between a semidwarf local variety, “Dee-geo-woo-gen”, and a disease-resistant gardening variety, “Tsai-yuang-chung”. In the late 1960s, a semidwarf variety, “IR8”, was similarly bred from a cross between the semidwarf variety “Dee-geo-woo-gen” and an Indonesian high-quality long-culm rice variety, “Peta”, at the International Rice Research Institute (IRRI), Philippines. This variety was called “miracle rice”, as it dramatically improved the yield per unit area. The spreading of “miracle rice” relieved the food crisis in Asia and gave rise to the “green revolution”. The gene that contributed to the high yields of both Taichung Native 1 and IR8 is the semidwarf gene sd1 derived from Dee-geo-woo-gen. However, to date, only the approximate chromosomal locus of the sd1 gene has been determined (Maeda et al., Breeding. Science 47: 317–320, 1997).
In general, plants, particularly useful agricultural crops such as rice, must be cultivated under well-fertilized conditions (i.e., nitrogen-rich conditions) if their yields are to be enhanced. In such conditions however, plants become so tall that they are apt to be blown down by typhoons and such, thereby resulting in the reduction of yields. One method for resolving such problems is to dwarf plants and cultivate them under well-fertilized conditions. The sd1 gene dwarfs plants only slightly and does so without reducing the number of tillers or size of the grain, or decreasing the number of seeds. It also prevents plants from falling down under well-fertilized conditions and improves plant shape. In this manner, the sd1 gene results in phenotypes different from those induced by the already-known dwarf genes, d1 and d61 (Ashikari M. et al., Proc. Natl. Acad. Sci. USA., 96: 10284–10289, 1999; Yamamuro C. et al., Plant Cell, 12: 1591–1605, 2000). This improvement in resistance to falling down enables plant cultivation under fertilized conditions; likewise, the improvement of plant shape enhances substance producing capability and distribution ratio of assimilation products into grains and seeds. To date, utilizing these properties, many rice varieties, including IR64 having the largest rice planted area in the world, have been conferred with phenotypes induced by the sd1 gene through back cross to breed novel rice varieties.
On the other hand, with a recent explosive increase in population, grain yields need to be further increased by 50%, so there is an urgent need to breed varieties of various useful crops with high yields. Therefore, in order to increase the yield of various plants, useful agricultural crops, including rice in particular, it is advantageous to utilize the sd1 gene that induces stable yield increases under fertilized conditions. However, the isolation and identification of the sd1 gene of plants including rice has not yet been reported.